Description. Thallus up to several centimeters in width and capable of expanding to at least 10 cm by replication by division, squamules 1-3 mm wide (rarely areoles), up to 1.5 mm thick (including stipe), sometimes imbricate. Upper surface yellow, epruinose, smooth, often with fissures, eventually replicating by division. Lower surface white or brownish, ecorticate. Epicortex thin, up to 1.5 µm, continuous, or absent. Cortex 50-90 µm tall, upper layer yellow up to 30 µm thick, lower layer hyaline, cells mostly round, 3-5 µm wide. Algal layer 100-130 µm tall, upper and lower surface usually even, dense, algal cells ca. 10-14 µm wide. Medulla 200-300 µm thick, narrowing into mycelial base or stipe up to 300-500 µm tall, obscure with crystals and gelatinization, hyphae mostly 2 µm wide, thin-walled. Apothecia often rare, but sometimes abundant, one to seven per areole, 0.2-1.0 mm wide, disc usually yellow, epruinose, rarely with ontogenic remnants of cortex on surface. Parathecium expanding around the disc up to 100 µm, often rising slightly above adjoining cortex, same color as thallus, IKI-. Hymenium (80-)100(-130) µm high, epihymenium yellow to reddish yellow, 10-30 µm high, the yellow pigment sometimes extending downward into hymenium 40-50 µm, paraphyses 1-2 µm wide, apices unexpanded, hymenial gel IKI+ red or blue turning red (hemiamyloid). Asci 80-100 × 14-20 µm, clavate, several hundred ascospores per ascus, ascospores ellipsoid, mostly 3-5 × 1.0-2.5 µm. Subhymenium 50-90 µm tall, persistently IKI+ blue. Hypothecium narrow, 10-20 µm wide. Pycnidia common, ostiole usually visible, globose to ovate, 60-120 µm wide, conidiogenous cells ampliform 14-20 × 1.0-1.5 µm, conidia variable, 1-2.5 × 0.5-1.5 µm.
Chemistry. Rhizocarpic acid (major). Not producing other secondary metabolites. Spot tests negative.
Distribution and Ecology. Common at least in Mojave and Sonoran Deserts to northern New Mexico, on siliceous rock in full sun.
Differentiation.Acarospora radicata differs from A. socialis usually having up to 7 apothecia instead of usually one and in occurring in desert habitats while A. socialis occurs along the Pacific coast. In some parts of California, the two species may be sympatric or hybridized. Acarospora radicata differs from A. organensis in having a lower hymenium (80–130 vs. 135–170 µm). Depauperate, areolate specimens may be hard to determine.
Discussion. In the Sonoran flora Acarospora radicata was treated as a synonym of A. socialis. Specimens of A. socialis in herbaria are often A. radicata especially if not collected along the Pacific coast.